This invention relates to a fuel cell and more particularly, to a cell structure of a fuel cell stack.
FIG. 1 is a partial cross-sectional view illustrating a typical cell structure of a fuel cell which is disclosed, for example, in Japanese Patent Publication No. 59-152, Japanese Patent Laid-Open No. 59-66067, and Japanese Patent Laid-Open No. 58-68881. In this figure, catalyst layers 2 and 3 are disposed on both faces of an electrolyte matrix 1, electrode substrates 4 and 5 being disposed on the backs of catalyst layers 2 and 3, respectively. These catalyst layers 2 and 3 and substrates 4 and 5 make up fuel side and oxidant side electrodes 6 and 7, respectively. The substrates 4 and 5 are made of a porous material such as carbon fibers. Gas separators 10 (also called interconnectors) made of materials such as impermeable dense carbon plate are disposed on the back sides of the substrates 4 and 5. On the portions of the gas separators 10 adjacent to the substrates 4 and 5, gas channels 11 and 12 are provided for fuel and oxidant gases, these channels crossing each other orthogonally. Fuel and oxidant gases are supplied to the gas channels 11 and 12 and then reach the whole area of the substrates 4 and 5, where the gases are diffused to reach the catalyst layers 2 and 3. Then, the fuel and oxidant gases at the catalyst layers 2 and 3 react with each other and generate power through the electrolyte matrix 1. At this time, non-reacted excess gases and water vapor which is a reaction product are exhausted to the exterior of the fuel cell through the gas channels 11 and 12. Moreover, the exhausted gas will contain an electrolyte which is included in the matrix 1 and electrodes 6 and 7 in a vapor state determined by the operating conditions of the fuel cell.
At both faces of both ends of the matrix 1 (only one end being shown) wet gas seals 8 and 9 are disposed to be adjacent to the end surfaces of the substrates 4 and 5, respectively. These wet gas seals 8 and 9 are for preventing the fuel and oxidant gases in the gas channels 11 and 12 from leaking through the porous substrates 4 and 5 to the outside of the fuel cell.
The conventional fuel cell is constructed as described-above, and therefore, the electrolyte is only held in the matrix 1, catalyst layers 2 and 3, and wet gas seals 8 and 9. When the fuel cell is operated for long periods of time, a shortage of the electrolyte is caused by evaporation or dispersion thereof, whereby only electrolyte impregnated and held in the wet gas seals 8 and 9 would be supplied to the matrix 1 and the catalyst layers 2 and 3. This results in that frequent replenishment of the electrolyte from the exterior of the fuel cell is required. Moreover, the volume of the electrolyte varies considerably according to the operating conditions of the fuel cell such as operation pressure, operation temperature, gas utilization ratio, and initial level of the electrolyte. However, conventional fuel cells do not have the ability to absorb such volume changes of the electrolyte. For example, when the cell size is large, even where an outer reservoir is connected to the fuel cell, the expanded portion of the electrolyte will overflow to the catalyst layers 2 and 3, substrates 4 and 5, or gas channels 11 and 12 before reaching the outer reservoir. Because the distance the electrolyte travels in the matrix 1 to the outer reservoir is long, "flooding" of the fuel cell will result, whereby performance of the fuel cell will be lowered.
The minimum thicknesses of each component of the conventional cell are as follows: gas separator 10 is 0.6-0.8 mm, electrode substrates 4 and 5 are 1.8-2.0 mm, fuel side electrode is 0.06-0.1 mm, air side electrode is 0.1-0.15 mm, and gas matrix 1 is 0.15-0.25 mm. Therefore the minimum thickness of the unit cell is 4.5-5.3 mm. Therefore, in piling a plurality of unit cells there arises the problem that the total size of the fuel cell become large and the space factor in relation to output of the fuel cell is poor.